Shading is one of the most underestimated factors in solar projects. A chimney, a tree or a parapet wall that was dismissed during the site visit as "a bit of shade in the afternoon, nothing serious" can affect not only the panel it falls on but the entire string that panel belongs to. In this guide we look at why shading has such a large electrical impact, the different types of shading, how to carry out a proper shade analysis step by step, and the mistakes that are most often made in the field.
How does a shaded cell affect a string?
A solar panel is made of cells connected in series. In a series connection, the same current has to flow through every cell. When one cell is shaded, the current it can produce drops and it can no longer carry the current of the other cells. The shaded cell becomes reverse biased and, instead of passing current, starts dissipating energy as heat. If this continues for long periods it can create a hot spot and, over time, damage the cell and the backsheet.
The role of bypass diodes
To limit this problem, a panel's junction box usually contains three bypass diodes. Each diode protects a group of cells (a substring). When the current in a substring drops significantly because of shading, the corresponding diode starts conducting and the current bypasses that substring. The panel keeps working, but the voltage of that substring is lost. In a panel with three diodes, bypassing one substring means losing roughly one third of the panel's voltage.
Impact at string and MPPT level
Because the panels in a string are connected in series, the string current is affected by the weakest link. When bypass diodes activate, the string's current-voltage curve develops more than one peak. The inverter's MPPT (maximum power point tracking) algorithm may not always find the highest of these peaks, which is why some inverters offer a global MPPT function that scans the curve periodically. Strings connected in parallel to the same MPPT input share a common operating voltage, so shading on one string can force the other to operate at an unfavourable point as well. We cover this in more detail in our stringing and MPPT guide.
In short, a shaded area of just a few percent on one panel can turn into a disproportionate production loss. The purpose of shade analysis is to see exactly this disproportionate loss before installation and to correct the design accordingly.
Types of shading
Near shading
This is shading caused by obstacles close to the panels: chimneys, vent stacks, antennas, roof exits, parapet walls, air-conditioning outdoor units, lift machine rooms and nearby trees. Near shading has sharp edges, falls on part of a panel and moves quickly during the day. It has the largest electrical impact because it affects individual cells and substrings.
Far shading (horizon shading)
Mountains, hills or tall buildings in the distance can block the sun completely, especially in the morning and evening. Far shading usually affects the whole array at the same time, so it does not create electrical mismatch, but it shortens the daily production window. It should be included in the yield estimate through a horizon profile. The option to use horizon data in tools such as PVGIS is useful here; see our PVGIS guide for details.
Seasonal shading
The height of the sun in the sky changes throughout the year. At noon in summer the sun is high overhead; in winter it comes from much lower. A neighbouring building that casts no shadow at all in June can shade a significant part of the array in December. Carrying out the site visit in summer is one of the most common reasons why winter shading is missed.
Inter-row shading
In ground-mounted plants and on flat roofs, tilted panel rows shade each other. As the sun gets lower, the shadow of the front row gets longer. For a south-facing row at solar noon, the shadow length is approximately obstacle height / tan(solar elevation angle). With example values: at a latitude of 38°, the noon solar elevation at the winter solstice is about 90° − 38° − 23.45° = 28.55°. Since tan(28.55°) ≈ 0.544, if the top edge of the front row is 1 m higher than the bottom edge of the row behind it, the noon shadow is about 1 / 0.544 ≈ 1.84 m long. In the morning and evening the sun is lower, so the shadow is even longer. We look at row spacing separately in our article on row spacing in ground-mounted plants.
A proper shade analysis, step by step
- Record obstacles on site. Measure the position of each obstacle, its distance from the panels and its height. Record heights consistently against the ground or roof reference, not the panel plane. Add photos and a sketch.
- Note far obstacles and the horizon. Identify hills or tall structures to the east, south and west that could block the sun.
- Build a scaled model. Roof surfaces, tilt and orientation, obstacles and panel layout should come together in one model at the same scale. A layout done by eye will not show correctly which panel the shadow falls on.
- Simulate the critical dates and times. At a minimum, check morning, noon and afternoon around the winter solstice (December), the equinoxes (March and September) and around the summer solstice (June). Winter mornings and winter late afternoons usually show the worst case.
- Evaluate shading loss per panel. Work out which panel is shaded, in which period and to what extent. Leaving permanently shaded positions empty is often the most economical solution.
- String according to shading. Group panels with a similar shading profile in the same string, and connect groups with different orientations or different shading profiles to separate MPPT inputs. Where needed, evaluate panel-level optimizers or microinverters together with their cost.
- Carry the result into the yield estimate. Shading loss must be reflected in annual production and the payback calculation. A production estimate that ignores shading is one of the most common causes of disputes with customers after installation.
Common mistakes
- Only looking at noon: Shadows are shortest at noon. Morning and late afternoon hours, especially in winter, can be far more critical.
- Visiting the site in summer and forgetting winter: A roof that looks shade-free in summer can look very different in winter.
- Ignoring parapets and small obstacles: A 40–50 cm parapet can shade the bottom cells of the first row for long periods when the sun is low.
- Putting shaded and unshaded panels in the same string: This can make the unshaded panels operate below their potential as well.
- Connecting different orientations to the same MPPT input: East and west surfaces receive light at different times; separate MPPT inputs are usually the more accurate solution.
- Leaving shading out of the yield estimate: The shading you see in the design should also show up in the kWh figure in the proposal.
- Ignoring dirt, snow and dust: Dirt collecting along the bottom edge of low-tilt panels can behave like permanent partial shading.
Live with the shade or change the layout?
Not every shadow is an unavoidable loss. In most projects the solution is one or more of the following: leaving permanently shaded positions empty, shifting the layout away from the obstacle, increasing row spacing or reducing tilt, restringing according to the shading profile, or using panel-level power electronics. Which option is right depends on comparing the lost production with the extra cost. That comparison is only possible when the shade analysis produces a numerical result.
Shade analysis with PVAGE
In PVAGE's roof design screen, you draw to scale on a satellite image or photo and place panels automatically or manually. You define obstacles such as chimneys and antennas, and shading objects such as neighbouring buildings and trees, together with their heights. With the sun simulation by month and hour, you see when the shadow falls on which panel and follow the shading loss per panel. Automatic and manual stringing show the distribution per inverter and per MPPT, so you can adjust the string layout to the shading profile on the same screen. For ground-mounted projects, the mounting structure module offers row spacing adjustment and inter-row shading simulation by month and hour. See the shade analysis feature page for details.